This invention relates to droplet deposition apparatus.
In particular the invention is concerned with a printer or other droplet deposition apparatus in which an acoustic pressure wave is generated by an electrical signal to eject a droplet of the liquid (e.g. ink) from a chamber. The apparatus may have a single such chamber, but more typically has a print head with an array of such chambers each with a respective nozzle, the print head receiving data-carrying electrical signals which provide the power necessary to eject droplets from the chambers on demand. The or each chamber is bounded by a piezo-electric element which is caused to deflect by the electrical signal, thereby generating the acoustic pressure wave which ejects the droplet. Reference is made to our published specifications EP 0277703, U.S. Pat. No. 4,887,100 and WO91/17051 for further details of typical constructions.
It is customary in such apparatus that the voltage of the electrical signal required to eject a droplet is minimized; lower voltages permit the driving circuitry to be simplified and/or reduced in cost. Furthermore, the heat generated during operation of the print head, which is proportional to V2 in both the print head and its driving circuitry, is also minimised. Excessive heat generation is to be avoided because it affects the fluid properties of the ink, leading to inaccuracies in printing, especially if there are significant variations in temperature between different chambers of the print head. Such variations occur when one chamber is operating significantly more frequently than another, eg when one is printing a dense area of an image and the other a significantly less dense area. To this end, a soft (donor-doped) lead zirconate titanate (PZT) material often is the preferred piezo-electric material. Soft PZT has a high piezo-electric activity; that is to say a given voltage will produce a relatively large physical deformation of material, which is particularly effective in ejecting the liquid droplet from the chamber.
Further reductions in drive voltage can be achieved by arranging the piezo electric material in xe2x80x9cchevronxe2x80x9d configuration, as described in the context of an xe2x80x9cend-shooterxe2x80x9d print-head in our EP-A-277703. Alternatively or in addition, the print head can be configured as a xe2x80x9cside shooterxe2x80x9d as described in our WO91/17051. Both of these designs halve the drive voltage for a given droplet ejection performance relative to an xe2x80x9cend-shooterxe2x80x9d design employing a monolithic piezo electric element; adopting both of them reduces the drive voltage by a factor of four.
By xe2x80x9cend-shooterxe2x80x9d we mean a configuration in which the nozzle is at the end of elongated chamber, the piezo electric material being disposed along the sides of the chamber. In a side-shooter, the nozzle is instead disposed in one of the long sides of the chamber which is not bounded by piezo electric material. In a xe2x80x9cchevronxe2x80x9d design a longitudinal side of the chamber is bounded by piezo electric material having oppositely-poled regions extending longitudinally of the chamber, so that application of the electrical signal deforms both regions of the material of the same direction into a chevron shape, when viewed in cross-section.
Whilst the foregoing expedients may be thought to offer both low drive voltages and low heating effects, they have a serious disadvantages, namely that compared to a monolithic end-shooter, both of them approximately double the capacitance of the chamber wall, as seen by the drive circuit. A chevron side shooter design thus has four times the capacitance of a comparable monolithic end-shooter. High capacitance has two effects. Firstly capacitance heating effects are increased with the disadvantages already discussed, and secondly the high capacitance increases the time constant (RC) of the device. The waveform of the driving electrical signals is preferably as close as possible to a square wave, so that the sharpness of the acoustic pressure waves is maximised. A large time constant increases the rise time of the circuit in response to a step change, with the result that its ability to produce an effectively square waveform at high frequencies is compromised. The frequency of the drive signals thus has to be limited, thereby reducing the speed at which the printer can be operated. This is particularly important in variable density (xe2x80x9cgrey scalexe2x80x9d) printers, in which each deposited droplet is made up of a controllable numbers of smaller sub-droplets produced at very high frequency.
The preferred embodiments of the present invention are directed to this problem.
The invention provides a droplet deposition apparatus comprising a liquid droplet ejection nozzle, a pressure chamber with which the nozzle communicates and from which the nozzle is supplied with liquid for droplet ejection, a wall of the chamber comprising a acceptor-doped piezo electric material deformable upon the application of an electrical signal to eject said droplet from the nozzle.
Preferably the material has a hysteresis loss (tan xcex4) of substantially not more than 0.05 at the voltage of the applied electrical signal.
The hysteresis loss tangent is given by
Tan xcex4=∈xe2x80x3/∈xe2x80x2
Where ∈xe2x80x3 is the imaginary part of the permittivity and ∈xe2x80x2 is the real part.
Preferably the material has a figure of merit (as herein defined) of between 15 and 30, and preferably of about 25.
By xe2x80x9cfigure of meritxe2x80x9d we mean the quantity
d15/(S55xc2x7∈0)1/2
tan xcex4=∈xe2x80x3/∈xe2x80x2
where d15=shear strain/electric field piezo electric constant
S55=electric shear compliance
∈0=permittivity of free space
Examination of a range of PZT materials has shown the general trend that high figure of merit is associated both with high loss tangent and high relative permittivity.
As already indicated, the invention is particularly suitable for apparatus in which the piezo electric material is deformed in shear mode, the apparatus having one or preferably both of the xe2x80x9cside shooterxe2x80x9d and xe2x80x9cchevronxe2x80x9d configurations.
The preferred piezo electric material for use in the invention is an acceptor-doped PZT such as that sold by Morgan Matroc under the designation PC4D.